The present invention relates to a semiconductor device and method for manufacturing the same, and more specifically, to a semiconductor device in which moisture-proofness of an airtightly sealed semiconductor package is improved and a method for manufacturing the same.
In a semiconductor chip such as an IC, LSI or VLSI, a highly-integrated circuit surface tends to be affected by the change in ambient temperature of humidity, or fine dust. Further, since a semiconductor chip itself tends to be broken by mechanical vibrations or shocks, it is used as a semiconductor device together with a package for covering the semiconductor chip.
The package type is roughly divided into an airtight seal type and a resin mold type. In the former, the airtight seal type, an airtight space is formed in the central portion of a package member, and a semiconductor chip is arranged within the airtight space. As a material for such a hollow package, ceramics having a high moisture-proofness are generally used, but synthetic resins which have low cost and are easy to work with are also used.
In a semiconductor device using such a hollow package made of synthetic resin, a semiconductor chip is secured to a bonding surface (a chip bonding surface) of a hollow portion by means of adhesives and is electrically connected to a lead frame by means of a bonding wire. Opposite ends of the lead frame are opened to an inner side and an outer side of the package.
The top surface of the hollow package is airtightly sealed by securing a cover member or lid such as a transparent or opaque synthetic resin plate, a glass plate or the like thereto by adhesives.
In the case where such a hollow package made of synthetic resin is used, there arises a problem in that even if the cover member is secured by means of adhesives, a fine amount of moisture enters into the package as time passes. As a result, electrodes of the semiconductor chip or internal circuit becomes corroded to lower the function thereof, and finally result in an unusable condition.
Even if careful seal means is applied to an adhesive portion of the cover or a seal portion of the lead frame which are contemplated to be an entry route of moisture, it is still not possible to prevent entry of moisture after passage of time, a countemeasure of which has been worked out.
The present inventors have repeatedly made studies to clear up the cause of entry of a fine amount of moisture into an airtightly sealed semiconductor package. At the outset, it has been assumed that a route of entry of moisture into the semiconductor package is from the adhesive portion of the cover member or the seal portion of the lead frame.
Therefore, tests were repeatedly carried out to clear up the cause from that field.
However, it has been found that the entry of moisture from the adhesive portion of the cover member or the seal portion of the lead frame possess is not much of a problem since various countermeasures are taken. However, the entry of moisture into the semiconductor package principally results from the moisture which transmits through a molded body from the lowermost surface of the package, that is, the bottom surface of the hollow package.
It is to be noted that the entry of moisture into the semiconductor package was tested in the following manner.
A package sealed by a transparent cover member is put into a commercially available pressure cooker tester (PCT) and heated and pressurized under the conditions of temperature (121.degree. C.), humidity (100% RH) and gauge pressure (1 kg/cm.sup.2) for a predetermined number of hours, after which it was removed to examine if condensation was formed caused by moisture entered internally of a transparent cover member under normal temperature.
Those which have no condensation are continued to be heated and pressurized for the predetermined number of hours until condensation appears.
Accordingly, in the present invention, the quality of the moisture-proofness of the package was determined by the length of heating and pressurizing time until condensation appears in the inside of the transparent cover member.
It is an object of the present invention to provide a semiconductor device of an airtight seal type having an excellent moisture-proofness which can effectively prevent an entry of moisture.